Mince the garlic and grate the fresh ginger, then whisk together the teriyaki sauce and sesame dressing in a bowl until combined. This creates your flavor base and ensures the aromatics are ready to distribute evenly. Cut the chicken breasts into 1-inch chunks for even cooking, then chop the red onion into similar 1-inch pieces to match the chicken size. Slice both bell peppers into 1-inch pieces as well. Having everything prepped and your sauce ready will make assembly quick and efficient.
Preheat your grill to medium-high heat (around 400°F if using a gas grill, or until coals are hot on a charcoal grill). While the grill heats, lay out 4 sheets of heavy-duty foil on a clean work surface. Divide the chicken chunks, bell peppers, red onion, and pineapple chunks evenly among the four sheets, creating a loose mound in the center of each. I like to arrange the chicken on the bottom with vegetables and pineapple scattered on top so the sauce coats everything evenly.
Pour approximately 1/4 of the sauce mixture from Step 1 over each foil packet, drizzling it evenly over the chicken and vegetables. Fold the foil sheets up and over the ingredients to create sealed packets, crimping the edges tightly to trap the steam inside. Make sure the seals are secure so no liquid escapes during grilling—this steam is essential for cooking the chicken through and infusing the flavors.
Place all 4 foil packets directly on the grill grates and close the lid. Grill for 7-8 minutes, then carefully flip each packet using tongs (the foil will be hot!). Continue grilling for another 7-8 minutes until the chicken is cooked through—you can carefully open one packet to check that the chicken has no pink inside. The total time is 14-16 minutes, and the foil creates a steaming effect that keeps everything moist while the grill adds subtle char flavor.
Carefully remove the foil packets from the grill using tongs and place each one on a serving plate. Open the foil packets slowly to release the steam, then sprinkle each packet with fresh cilantro and toasted sesame seeds. Serve immediately while hot—the foil packets keep everything contained and make for an impressive presentation right at the table.